5.3L Swap Hard Start and Sputtering

[Green67Stepper]

I have a '99 5.3l LS swap in my 67 C10 with about 250-300 miles on the swap so far. It runs and drives fine overall, but i do have a few issues.

1: It never wants to start right up. It typically needs a slight touch of the throttle to start and usually wants to idle down and die if you dont gradually ease off. Once its idling, its fine, usually around 550 rpms. At Key On, Fuel Pressure surges to 58-60 psi and then stops and holds at 49-50 psi. It also idles around 49-50 psi.

2: Under normal driving, it runs great. Anytime its at 50% throttle or more though it wants to hesitate and sputter.


I have pretty much cleaned all the sensors with MAF cleaner (MAF, IAC, MAP) i have also removed the throttle body and Fuel Pressure Reg and cleaned them. I have tested the Fuel Injectors and they all work. I also removed them all and cleaned them with some Carb Cleaner. I unplugged all the plug wires at the coils and each one was sparking. I unplugged the MAF and it really runs bad unplugged, so i assume that means the MAF is working. The entire fuel system is new, lines, tank, pump, filter. I have also run Seafoam through the fuel system and vacuum line to clean injectors, etc.

Where should I go from here?

[ls1nova71]

Kind of sounds like the IAC isn't working quite right. Do you have another one you can throw on it and see what it does? Or do you have a scanner that shows IAC counts?

[BR3W CITY]

How modded is the motor?

Mechanically speaking; iac, vacuum leak, clogged fuel filter are worth looking at.

Is it tuned? Startup issues and idling down can be related to ASE, commanded idle, idle modifiers for trans and a/c.

Actually, running bad with the MAF unplugged indicates that the vehicle isn't running well in speed density. When the MAF is disconnected, it forces the ECM to SD and uses the VE tables for fueling without correction from the MAF.

[Green67Stepper]

Originally Posted by ls1nova71

Kind of sounds like the IAC isn't working quite right. Do you have another one you can throw on it and see what it does? Or do you have a scanner that shows IAC counts?

I have an 99 Sierra with a 6.0l that i could swap parts from. I don't have a scanner, but i am running a bluetooth OBD2 app on my phone. Not sure if i can pull that kind of info from it though. It hasn't thrown any codes.

Originally Posted by BR3W CITY

How modded is the motor?

Mechanically speaking; iac, vacuum leak, clogged fuel filter are worth looking at.

Is it tuned? Startup issues and idling down can be related to ASE, commanded idle, idle modifiers for trans and a/c.

Actually, running bad with the MAF unplugged indicates that the vehicle isn't running well in speed density. When the MAF is disconnected, it forces the ECM to SD and uses the VE tables for fueling without correction from the MAF.

The motor is bone stock. Just new plugs/wires, generic CAI and Shorty Headers. Nothing internally has been done.

I will swap the IAC off my other truck and see if that helps. I have checked for a vacuum leak and don't hear one anywhere. IDK how the fuel filter could be clogged since everything is new, but i'll check it anyways. its easy to get to on the frame rail.

It has been tuned, just a basic bench tune for a LS Swap. IDK what all was changed, but i thought it was just clearing VATS etc based on what he told me. I'm also not running A/C.

[mongocanfly]

that (basic bench tune) might be an issue as well...when my ecm was done their idea of a basic tune was for a 4.8 tune...and mine is a 6.0..but mine wasn't acting like yours is....but when I got it dyno tuned it ran much better..and picked up a bunch of hp

[Green67Stepper]

So i swapped the IAC sensor from my Sierra and that definitely helped with the startup issues. If fires up each time now with no throttle. I'll be getting a new one this week, and putting this one back in my Sierra. It is still running rough under heavy throttle and seems to be misfiring/sputtering. Its nothing horrible so i think i may just drive it until I can get it dyno/street tuned. Hopefully thats all it needs.

[68c10airstream]

Can you get a pic of the cold air intake??

[Green67Stepper]

Originally Posted by 68c10airstream

Can you get a pic of the cold air intake??

Here ya go.

[68c10airstream]

looks good. Some cai are a mess with a flexible duct, etc which causes turbulence. You stated that you cleaned the maf, are the resistors shiny like new?? Both the incoming and backside of the resistors should be shiny and clean like new. Look at them with the sun shining down at them helps, my eyes are starting to change.

[Green67Stepper]

Well I thought the new IAC sensor was going to fix my problem with the hard start. It did for about a day, but once the truck ran for a while its back to doing what it was before. SOMETIMES it will start without throttle but 9 out of 10 times it needs some to start and I have the feather off of it so it won't die.

One thing I did notice is my Volt Meter. It moves around a lot during the RPM range, while my DD's, 03 Yukon and 99 Sierra, are a constant 14v no matter what. When Key is on its around 10v while fuel pump is priming, then goes up to 12v.

Would this affect startup etc?

[mongocanfly]

just so you know. ..technically you don't have a cold air intake...cai are plumbed outside the engine bay...it looks like your filter is under hood...mines that way as well...

[homemade87]

How did you clean the injectors , just spray them with carb clean . If so this does nothing to clean them . With all the problems you are having you may want to revisit this .

There is a inexpensive way the cycle cleaner thru the injector . Take a look on utube and you will see what I am talking about . I made me one with the syringe and carb cleaner and it works pretty good . I do this to all engines that sit for a while before install to eliminate this problem right off the bat . These ls injectors are bad to gum up when sitting with the fuel today when they sit .

[Green67Stepper]

Originally Posted by homemade87

How did you clean the injectors , just spray them with carb clean . If so this does nothing to clean them . With all the problems you are having you may want to revisit this .

There is a inexpensive way the cycle cleaner thru the injector . Take a look on utube and you will see what I am talking about . I made me one with the syringe and carb cleaner and it works pretty good . I do this to all engines that sit for a while before install to eliminate this problem right off the bat . These ls injectors are bad to gum up when sitting with the fuel today when they sit .

I'll take a look at that. I sprayed them down with Carb Cleaner and then connected each one to the battery with some jumper wires and cycled then on and off a few times to run cleaner through them.

[homemade87]

Originally Posted by Green67Stepper

I'll take a look at that. I sprayed them down with Carb Cleaner and then connected each one to the battery with some jumper wires and cycled then on and off a few times to run cleaner through them.

If you cycled cleaner thru under pressure and had a good pattern spray then you should be good .

[Green67Stepper]

its been a while, but I wanted to update the thread. I figured out my hard start issue and I feel like an idiot. In short, I was having a issue with my ignition plug getting hot and melting the red and pink terminals. after replacing the plug I started tracking wiring and looking into what could be the cause. Finally I realized that when I wired up the motor, I never hooked up my 12v constant wires. I wired everything from my fuse/relays to 12v keyed power. So #1, I was pulling a ton off the one single keyed ignition wire and #2, my PCM never kept 12v constant. After running a new fused power wire from the battery for my PCM, it starts like a dream.

I am getting some sputtering/misfiring at high RPM/throttle, but I'm hoping its just the PCM re-learning everything and will clear up after getting some miles on it. I only drove 2-3 miles at most.